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Interview of Martin Schwarzschild by Spencer Weart on 1977 March 10, Niels Bohr Library & Archives, American Institute of Physics, College Park, MD USA, www.aip.org/history-programs/niels-bohr-library/oral-histories/4870-1
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Life of his father, Karl Schwarzschild; father's scientific relationships in Göttingen (Felix Klein, David Hilbert); move to Potsdam, 1909; relations with Potsdam and Berlin scientists (Albert Einstein, Karl Sommerfeld); father's Jewish background concealed. M. Schwarzschild's youth in Göttingen and Berlin; early education, interest in astronomy and mathematics. Undergraduate at Göttingen Universität (Hans Kienle, Richard Courant, Neugebauer), 1930-1933; graduate work at Gottingen Observatory, 1933-1935; his reaction to Nazism. Introduction to astrophysics (Arthur Eddington), interest in stellar interiors and stellar evolution; contacts with other astronomers from Gottingen Observatory (Otto Heckmann, Kienle, Rupert Wildt); comments on general relativity; interest in pulsating stars; leaves Göttingen, 1936. Postdoctorate at Oslo (Svein Rosseland); Jan Oort, Ejnar Hertzsprung; mechanical analog computer for computations in astrophysics and celestial mechanics; comments on development of theory of stellar interiors, 1939-1950. To Harvard College Observatory (Harlow Shapley), 1938; C. Payne-Gaposchkin, Bart Bok; comparison of European and American observational style, social scene; Barbara Schwarzschild's difficulties as female astronomer; contacts with S. Chandrasekhar and other astronomers. Tour of the United States; visits Mt. Wilson Observatory (Wilhelm Baade, Rudolph Minkowski, Edwin Hubble, Milton Humason), 1940; Shapley's relationship with Mt. Wilson staff. Harvard (Fred Whipple), 1938-1941; Shapley as a leader; astronomy summer school at Harvard; work on Cepheid variables in M3 (Bok, Chandrasekhar); overall impact on Schwarzschild of Harvard period. Columbia University (Jan Schildt, I. I. Rabi), 1940-1942; difficulties there; origin and funding of Thomas Watson Astronomical Computing Center; discussion of cosmology in the late 1930s; contacts with physicists (Enrico Fermi). In U.S. Army, 1941-1945; enters as private, teaches math to recruits; refuses invitation to Los Alamos; transferred to Aberdeen Proving Ground, dissatisfaction there; to officers training school, does bombing analysis for Italian campaign. Work relating to stellar interiors and evolution, 1938-1946; nuclear energy source ideas (Hans Bethe, Fermi); Eddington, Gerard Kuiper, Chandrasekhar, G. Keller; German astronomers during World War II (Ludwig Biermann). Discussion of wife's career and her role in his career. Early ideas about red giants (Öpik, Herman Bondi, Fred Hoyle), 1946-1950. Work on acoustic wave energy transport (R. Richardson, Gold); work on chemical composition differences in stellar populations. To Princeton University (Spitzer, H. N. Russell), 1947; Project Matterhorn (start of bomb and fusion projects); relationship with Russell. Stellar evolution work in the 1950s; computer work (John Von Neumann, Richard Härm), mid-1950s; collaboration with Allan Sandage evolving a stellar model, 1952; computing towards red giants; observational cluster work, 1951; ages, metallicity, and the Big Bang; beginnings of "astrophysical" cosmology. Evolution theory after late 1950s; effect of computers on theoretical progress; relation of evolution theory to cosmology; general comments on his work in stellar evolution; interactions with Robert Dicke; views on cosmology, general relativity. Need for better solar convection work leads to use of balloons (James Van Allen); post-Sputnik funding; on cooperation with industry and engineers; Stratoscope II (Bob Danielson, Spitzer). Years advising the National Science Foundation, President's Science Advisory Committee, 1959-1976, and National Aeronautics and Space Administration (Von Neumann), to 1969; The International Astronomical Union, 1964-1970; American Asronomical Society, 1967-1973. Informal advisor to various observatories: Kitt Peak National Observatory, Mt. Wilson-Palomar Observatories, Carnegie Southern Observatory. Recent work on galactic structure. Reflects on importance of ethical standards; his feelings about religion and nature.
I think I'd really like to spend a lot of time talking about your father if you're willing to. This is the first formal astrophysics interview that I've done. I've done a lot of informal ones. I think it's very appropriate that we should start out with Karl Schwarzschild. I really don't know quite where to start. First of all, do you have any personal, direct memories of him, or is it mostly through what you've heard in the family? Where do you have knowledge of your father from?
My direct personal knowledge is absolutely minimal. I was four when he died, and they are just typical for one's earliest memories — a couple of scenes of a basically irrelevant nature, but that at that time must have impressed me. But no scenes that make any characteristics of my father clear to me.
You must have heard a lot about him from your mother perhaps, or from relatives.
Probably most from my mother. But she was extremely cautious not to build up a picture in us children of my father as the size man that he was.
So as not to be overwhelming.
Not to be overwhelming, or, that we wouldn't be improperly proud of our father, which might be more the natural reaction of children, particularly (isn't that right) in the style of the Germany of that time when a real great professor was a major figure.
That had tremendous prestige.
Enormous prestige. To be the son of that kind of a figure can easily develop a high pride. His having died, I think, reduced the danger of feeling overshadowed by him — obviously by a very large factor. I do not know what my mother was thinking and feeling, but she was quite cautious not to talk about him all that much to us children. Somehow she must have talked enough, or described this or that little event enough, so that we did have a picture of him as a person. I think in a way to the degree that you can as a child be attached to a person whom in effect you have not met, we were attached to my father, but in that abstract sense.
That's interesting. And then you must have also heard about him I suppose from other people.
Yes. Again, I think people were cautious, as long as we were youngsters, not to in any way make my father's figure a condition limiting us in one way or another. It's really rather much later that people started talking more freely about my father to me. Indeed, I think some of the really impressive stories I heard only after I came to this country, and then on later visits back to Germany people were quite free. By that time I was a grown person and nobody worried anymore (isn't that right?) about the effects on me.
Let's see; I'm not quite sure what we should talk about, about your father. I think the main thing is to talk about things that we may not be able to recover from the papers that have been saved the published papers and the correspondence and so forth.  I guess the first thing of interest is: do you happen to know how he got interested in astronomy? What was it in his upbringing that directed him towards a career in astronomy or in science in general?
No, I have no direct knowledge, except I know that he was an extremely good student in high school. I can only suspect that he was clearly outstanding in high school mathematics, which in German classical schools leads into calculus, so there is some measure. Actually, he got interested in astronomy as a high school student. He first published a small paper when he was still a high school student. What interested my father in astronomy, I do not know. His first interests seemed to have been very much on the mathematical, theoretical side of astronomy.
He was, of course, one of the first people to be called an astrophysicist, and I wondered what there might have been that was special about his orientation that brought him to this.
That was not his very first introduction to astronomy. His very first introduction was very much classical.
On the mathematical side.
Right, from the French classical school. Up to his thesis…
That's true — I was jumping ahead a little bit.
Right. Whether there was a strong astrophysical impact on my father before he came to Gottingen, I do not know. In his Munich time, under Seeliger, I do not think so. I do not know when he first came very close to Sommerfeld, who was a lifelong companion and a leader into modern physics.
It might even have been at the time your father came to Gottingen where there was so much interaction between physics and mathematics and so forth.
I do not remember whether Sommerfeld was in Gottingen at that time or whether that was already started in the Munich time.  I just do not know. But in Gottingen obviously my father was surrounded by the most modern physics of that time. Both his scientific and social colleagues were among the physicists and the mathematicians during the Gottingen time. So he was exposed to modern physics as well as mathematics.
Do you know how he happened to get to Gottingen? He was appointed there so young.
No, I'm not at all sure. Possibly already at that time the Kultus minister of Prussia, to which Hanover belonged was Althoff, who may have been early impressed by my father. Whether he already had the essentially final say of that type of appointment, directors of institutions, within Prussia and really made the decision, I do not know. But obviously the proposal must have come from other sources, I do not know from where.
He'd been there a few years when he came to marry?
No, he asked my mother to agree to marry him after very few years in Gottingen, but my mother was far from decided. In fact, they only got married just at the time my father was to move to Potsdam to take the directorship there. So all through the Gottingen time, my father was unmarried.
I see, but courting your mother, as we would say.
Right, in a very restricted form. Gottingen was more proper than places like Berlin, Frankfurt or Munich would be — indeed, extremely proper. My parents hardly could see each other than in formal balls. Courting then was very difficult.
I see. Your mother was from a Gottingen family, was she?
My mother was from a Gottingen family. One of her great-grandfathers had been the chemist Wohler, the one who is often called the father of organic chemistry. One of her grandfathers was for practically his lifetime the mayor of Gottingen. Her father, Julius Rosenbach, was a surgeon and professor in the medical school of Gottingen, so she was entirely within the university social strata, and this stratification was a very very strong feature of Gottingen.
There must have been some problems because your father was Jewish, in that case.
Those problems were extremely strong at the time and played a fairly severe role in the question of my parents getting married. A part of my mother's family was straight anti-Semitic and entirely against this marriage. But also on my father's family's side there was some resistance, nothing very personal. But on my mother's family's side it took much sharper and more personal forms. But my father's father, for example, in his long-standing will, assigned a small reduction of inheritance to those sons of his who would marry a non-Jewish girl. In due course my father's mother learned to like my mother extremely much, even though their backgrounds were enormously different. I mean my father's parents were from the business world; and in very fashionable Frankfurt. Even though the Jewish community was still an entity and not integrated in any form, they participated in the fashionableness of a large and wealthy city, while my mother came from an extremely stodgy Victorian background. But as persons they grew to like each other quite enormously. My mother's mother was anti-Semitic and never really, I think, quite got reconciled. My mother's father, who was a surgeon as I said, was very free of any such prejudices, but had quite a hard time understanding my father, who was also fairly formal — more formal than the rest of his family — and was not I think what one would describe as an effective courter. Indeed, I know that he never kissed my mother until the final date, and after the first kiss they got officially engaged.
Very proper, I see.
Indeed, my mother loved to tell (when we children were older) that one of the subjects that my father always liked to persuade my mother to marry him with was to describe what a happy old couple they would be after he was retired. My mother just wanted to really feel sure that there was active love — isn't that right? Actually, their engagement occurred after a rather difficult discussion they had had, when my mother had said a straight no to my father. And then during the night she decided that she really couldn't live without my father and phoned him and suggested a walk. My father of course immediately agreed to it, and my father started, when they started the walk, in his usual way trying to persuade my mother, and my mother suggested that maybe they shouldn't talk at all. Then within very little time my father kissed my mother for the first time, and then my mother was convinced.
That there was something there.
Right. And then there was still terrible difficulties in the family, but my parents were entirely decided.
I have some personal sympathy for that kind of thing from my own family. But tell me: did this anti-Semitism also play a role in your father's reception in Gottingen in the professional circles, with the physicists and mathematicians there and so forth, or the astronomers for that matter?
I really do not know, you see, whom my mother knew best and whom I learned after my father's death to know well in Gottingen were the friends of my parents, and they obviously were not anti-Semitic — figures like the Hilberts and Karl Runge, a less well-known physicist, and many more; and as Prandtl, who was a close friend of my father's, both unmarried for many years in Gottingen, equally young and equally bright and starting new tracks. None of them had any anti-Semitic reactions. There may have been others, but those of course I would not know about.
Wouldn't know about it, and they wouldn't perhaps have considered it a thing to tell you, to talk about.
At least no natural family friendships developed, of course, and so I would not know.
I'm interested that these friends of your father were in fact people like Hilbert or Runge or whatever, people on the mathematical and physics side. What about the astronomers there? There must have been some other astronomers.
There were — none so outstanding, but I think some younger ones who were extremely attached to my father. I cannot remember now their names, but that may be my lack of knowing the whole group. As far as I know there were none who became later on truly outstanding
Do you know whether he encountered any resistance in bringing in astrophysics and so forth? In some places I've heard that there were some conflicts between the old mathematical astronomers and the astrophysics people.
Yes, but since my father after one year was made the director of the observatory, he was the Herr Professor. So the rest of the astronomers in the social structure of that time could hardly react very harshly. Actually, I think two older people, Professor Ambron and I think Professor Meyermann, were, though I'm not quite sure about their relative ages, more on the classical side. But I think they learned to like my father really well. I have the impression that my father easily made friends and overcame differences of evaluation of fields of science. But for him personally it was quite clear that the major scientific stimulus came from a much wider ground than just the observatory.
From the physicists, and I suppose internationally. You mentioned that when you came over to this country you began to hear stories from people about your father. This must have been from people of various countries.
Yes, but the stories that I first heard about my father here were all from the one trip that my father in 1910 made to this country.
I see. People still remembered that.
Oh, yes. That was a very famous occasion. The total number was not so large. It was the first officially international meeting which Hale called. Actually I think it was then called the International Solar Union, but actually they talked about all parts of astronomy. It was a long affair visiting several of the observatories. The main meeting was actually at Mount Wilson, but my father made a round trip to various places of interest. With all the interest and all the difficulties that he would encounter. One of his first stops was with Pickering at the Harvard Observatory, where in the first morning when he got up he couldn't find his shoes, which he had left out in the hall to be shined by the servants. And he walked around in his socks and found Professor Pickering shining my father's shoes. So there were quite many things to learn in this country, and those stories he wrote to my mother, who remembered them. And other stories I heard here.
I see. Did they remember his personality well?
Yes. They clearly enjoyed him enormously and admired him; they maintained that he liked to drink beer when he was thirsty. And indeed he is quoted by Adams in a description of that Congress as having walked up, not ridden up on a mule, to Mount Wilson — a new and exciting observatory then. But the moment he was up there, so Adams said, he asked for a beer.
Yes, you said that your father was somewhat formal and sober with your mother, but I get the impression that he had a rather lively personality overall.
Yes. I think the question of his inefficiency as a courter does not reflect his personality at all. Indeed, the Gottingen observatory was quite the focus of informal gaiety, in a very lively sense.
In terms of seminars and that sort of thing or also after hours?
Oh, very gay evenings that had nothing to do with astronomy.
At your father's house or at the observatory?
Well, you see, the director's house was a wing of the observatory. My father's study was on the ground floor, and his living quarters, particularly the large living room, was in the upper floor of that same wing. The parties apparently spread all through that wing. The gaiety of these parties is indicated by the fact that they used to go directly from the study out into the yard through the windows. And once one person forgot that he wasn't on the ground floor but on the upper floor and stepped out of the window. He broke a leg but nothing worse.
And, of course, in an observatory one will always have people around in the evening to draw on anyway.
That is right. There is also the story that came out in print. When my father moved to Potsdam, Hartmann, who had been on the staff in Potsdam, according to my mother's estimates had wanted to become the director of Potsdam. But he was moved to Gottingen, or was offered and accepted the directorship in Gottingen instead; he probably felt quite hurt. So he surveyed the Gottingen observatory the way my father had left it very severely, and in the first annual report described the extremely rundown state of the Gottingen observatory as my father left it. The last sentence said that in one of the telescopes, in the focal plane with a special illumination a transparency of the Venus of Milo was found.
That's very interesting. I suppose the physicists and so forth were drawn into this sort of thing also.
Oh, yes. That lively young social group included physicists. Prandtl for example, was very much there. My father played the piano. I forget now whether Prandtl did the singing or played the violin. I do not remember.
And yet somehow your father managed to do quite a lot of work while he was at Gottingen also.
Yes. Well, the house rules — joking house rules — were actually typed and hung up. And one of the rules was: when the director starts taking his shoes off, the guests are required to leave. So my father did not have parties long through the night.
I see. They had to get back to work the next morning or whenever.
He wanted to, yes.
I suppose in a way you've answered my next question: where his interest in quantum mechanics, as it was at the time — and relativity and so forth — came from. Evidently it came from these physicists in a direct way through personal contact.
Yes. I'm not very clear how far the mathematical group and the physics group were really integrated. There was a mathematics club, for example, led by Hilbert, and Klein — a very important person, substantially older than my father but of enormous influence. I think it was Klein, who was very anxious to get the part of physics and applied mathematics built up, that drew people like Prandtl, for example, and my father to Gottingen, probably more than Hilbert.
In connection with that, do you know whether your father had any particular interest in this idea of Klein's of applied mathematics and applied physics and so forth, which was kind of a bone of contention at Gottingen at that time?
No, I do not know my father's role in that lively movement, if you want to call it that, though he was clearly in the midst of that. I do not know what position he took, though even at my time much later as a student in Gottingen, stories of this problem were told, and obviously the problem continued to exist.
If we could jump ahead, are these stories relating to the time when you were a student or are these relating back to the time of your father, because I want to ask about both?
Some of the stories related back to the Klein-Hilbert time. Indeed, I remember finding a postcard that was a caricature of Klein's intent to bring industry into actual financing of the sciences, then a very unusual idea in Germany. On this postcard you see professors in their formal gowns, and businessmen with big money bags, coming from two directions and pairing up and marching along side by side.
I see — marching down the aisle, so to speak.
Right. At one time or another after very serious discrepancies maybe not between the main characters but younger groups — a Versuhnungsessen, a peace-making dinner, was arranged. And Klein asked Hilbert to give the main dinner speech. As I remember the story, Hilbert started his talk by saying, "There are some who say that there is a fight between applied mathematicians and pure mathematicians. That can obviously make no sense, none whatsoever; because applied mathematics and pure mathematics have nothing to do with each other." Which, of course, had exactly the opposite effect of what Klein wanted. I don't think that Hilbert ever understood the politics of what Klein wanted to achieve.
I wanted to ask you what was going on when you were an undergraduate, but to pursue your father a little farther, I'm curious whether you know what was connected with his leaving Gottingen for Potsdam. It must have been very difficult for him to leave this lively community.
It was. Indeed it's the one move that I have heard some astronomers in this country and one or two in Germany call a mistake. The feeling among them was that my father was just in the process of building up a school of astrophysics in Gottingen under very very favorable circumstances, and his taking the directorship in Potsdam, even though he was teaching in Berlin University, still did not make it natural for him to really lead a young group. I can see the point why historically speaking one might consider it a mistake. Why my father did accept the position I do not know. There's no question that the Potsdam position at that time was considered the highest astronomical position you could have in Prussia or Germany as a whole. Whether my father thought it a duty — possibly specifically from the point of view of furthering the standing of Jews in Germany — whether that played a role is pure speculation on my part. I've never heard my mother talk about his reasons for the decision to go to Potsdam.
He would have been the first Jewish director, wouldn't he, in Potsdam?
What about your father at Potsdam? Do you have any idea of how he related with the people there and in Berlin?
In Potsdam he inherited a group of astronomers, with fairly little chance of bringing new people. The one whom he brought directly with him from Gottingen was Hertzsprung.
I didn't know that.
Yes. Hertzsprung had joined my father, or wanted to come to Gottingen, and my father was so impressed with Hertzsprung's early work — essentially the Hertzsprung-Russell diagram — that he did find a position for Hertzsprung in Gottingen. Then when my father did go to Potsdam, arrangements were made that Hertzsprung could come too. But otherwise as far as I know, the staff was fairly set, and not a staff that then or later contained what one might call great astronomers. So I had the impression from what my mother described that the difference of scientific penetration between my father and most of the staff members was such as to be uncomfortable at times. But it still seems that my father managed to make a friend out of everybody, even though my mother does describe an occasion or two when my father was fairly scientifically blunt — not derogatory…
Telling people their mistakes.
Very explicit about what was scientifically right or not right. Without, according to my mother, at all realizing that he was rather hard on such occasions — which were purely of a scientific nature in my father's mind — on the people of less penetration.
I understand the feeling.
The Potsdam Observatory is somewhat separate from the town. It's something like half an hour's walk from the nearest tram station. And therefore it had the danger of a small group in which frictions arise. My mother had the strong feeling when my parents' arrived at the observatory in Potsdam that relations between people were not of the happiest. But both the temperaments of my parents — even though my mother was much more quiet in a general way and my father was much more outspoken and lively and much more direct — both apparently succeeded to make a much more cooperative group out of it.
What was your mother's character like?
My mother was basically extremely quiet; in her mind, very active. In school, as far as girls' schools used to go, she was the class leader: not necessarily in grades but a natural leader, she had the idea of what to do and the class followed.
She would not have been to university, this is in…
Oh, no. Indeed, she took some university courses, but then the university decided that ladies were not even to be auditors, and they were put out of the courses that they audited. There was no question about them being students. I think also my mother's family would have considered that quite improper. My mother, after going one year to a polishing school, I think in Weimar, was essentially to stay home and wait for a husband. And both she and her much younger sister were not at all of the right generation to fit that role anymore. So my mother essentially retired from the scene by helping my grandfather in his bacteriological laboratory, which he had at home, in which he worked at night. My mother found that interesting work, and she actually participated in the very early developments of that field without ever having a scientific background. It occupied her, but in a sense withdrew her even more and strengthened her natural tendency of being very quiet and quite withdrawn.
In Potsdam, however, did she interact with the people on the staff and so forth?
Apparently very much. In any case I know that after my father's death she had very close friends at the observatory. Indeed, immediately after my father's death for several years we spent our summer vacations at the Potsdam observatory, which was in one sense very hard for my mother and in another sense of course she felt that she belonged there.
It must have been hard for your mother also to leave Gottingen and go to Potsdam when all of her friends were there.
In one way, yes. By the time my parents married, my mother was entirely devoted to my father, and there was just no question and nothing but, I think, sheer happiness to follow him. She was quite horrified about the coldness of the director's building at the Potsdam observatory. It was really of a ghastly style, and it took my parents a while to find a small, comfortable room where they both put their writing desks and spent the evenings, my father working and my mother writing letters or whatever she did. In the relations between my parents those short years of my father's move to Potsdam until the outbreak of the (First World) War, which were really very few years, were personally an extremely happy time, which then of course very abruptly got changed for both of them by, my father going into the German army.
During that Potsdam period did they have much relations with the physicists in Berlin?
Yes, my father particularly, though my mother used to go, for example, to the academy meetings, which were partly a social event.
I didn't know that.
Ladies used to come and have their own luncheons, and then there were apparently dinners.
This would be the Berlin Academy?
Right, not the University.
The Akadamie der Wissenschaft.
Yes. My impression from my mother's description — and again even though she was from a very stiff upbringing in Gottingen she herself had little inclination toward stiffness — and the Berlin Academy ladies were of a style of being convinced of the importance of themselves, so that my mother always had to smile about them and I think sometimes let show that she did not feel the same. And I think in that sense my parents were extremely similar. I mean my father, from many indications I have heard, just could not stand being put on a pedestal. Hertzsprung once told me and quoted my father as saying, "I cannot work with somebody who is looking up to me".
Have you heard about your father from the Berlin people also? Did he have close relations? I do not know — so many people were there. Einstein was there during your father's time?
He came just about the same time, didn't he?
That is right. But of course I do not know anything from Einstein about my father. I have met Einstein just a couple of times for short periods
You don't know whether your father's interest in relativity may have reflected some personal relations?
Not very close ones. But also the number of years were few (isn't that right?) and the separation from Berlin to the observatory was quite a trip. There was an appreciable practical separation. By temperament, I think my father and Einstein also were quite different. A man like Sommerfeld I think was much closer in temperament to my father.
Is that so? In what way?
Oh, outgoing, not athletic but they both loved hiking. They loved mountain climbing. Indeed, for quite a number of years each summer, Emden, my father's brother-in-law, who also was in astrophysics, and Sommerfeld and my father did spend a week or two together in the Alps for real mountain climbing with guides and on ropes. Even though all three of them were very fit and very enthusiastic for this kind of physical activity, there were apparently occasions when they got very deeply, right in the middle of the mountains, into scientific discussions. There was one case where the artist brother of my father was with them and found that they made the guides stop in the middle of a glacier crossing and let the ropes slack so that the three scientists could get together and make a drawing in the ice to come to a conclusion on some scientific argument. My uncle, the artist, was so angry about it that he never went again with those three scientists. My father did go skiing in the Harz mountains near Gottingen quite regularly. That type of physical energy and satisfaction was natural with his very outgoing, lively character. I have the impression that Sommerfeld was more of that kind than Einstein.
Speaking of your father being outgoing and so forth, there was another question I wanted to ask: he had a kind of commitment to popularizing astronomy, and I wonder if you had any idea of where this came from.
I have only a suspicion but not a real knowledge. I think that the Frankfurt community, Jewish community, was really cultured and maybe consciously wanting to further their cultural level.
So this may have been something that he brought with him from a very early age.
I rather have a feeling that it came from that background. Also it was not at all contrary to my father's personal characteristics. But I think the impetus may have come from that background.
I want to ask you a few questions that I've been asked by Jeff Crelinsten, who's interested in the history these times and relativity and so forth. Do you know whether there was any particular arguments that your father got involved in over relativity, for instance, before the real problems developed over Einstein and so forth? But did he encounter resistance when he started to talk about relativity?
Very definitely. That is the older leading German astronomers — naturally by age and the time of their upbringing — were, by the time that general relativity came about, conservative. Quite specifically, my father's main teacher, then quite an old but still active astronomer, Seeliger, was very very skeptical — I might put it even stronger — about general relativity. But I think he was one of the most reasonable spokesmen of what I gather was a very common reaction in astronomy, excepting a few very young people, younger than my father. My father actively followed general relativity, and the beginning was already in his Gottingen time even though the definitive papers of Einstein came only later in my father's Potsdam time.
In Gottingen it would have been by way of Minkowski perhaps?
I do not know, except that Minkowski was there. My father's interest developed apparently very early, and indeed among the later letters are one or two which apparently react to some fairly sharp attacks on some of the younger astronomers, younger even than my father at that time, specifically Freundlich, from the older group, and my father was clearly the middle man to whom the astronomers talked. Even in correspondence when my father was in the army and away from Potsdam, some such talk persisted. in that one letter (to Seeliger, 1916) that we found (isn't that right?), where my father expressed that he had fallen in love with general relativity, he still had to admit that some of the over-enthusiasts like Freundlich were going too far and started interpreting very unsafely astronomical phenomena in terms of general relativity, a criticism which later on proved justified. It turned out very very much harder to find relativistic effects in astronomy.
Another question about the Gottingen period — something I'm not very clear about: and that is the distinction between the observatory and the university. Was there a distinction or was it just like a university department?
It was a university department. As far as I know, the observatory was just the building for the astronomy department. I don't remember whether it was called the Gottingen Observatory or the Gottingen University Observatory. But there's no question that everyone in the observatory was in the university. It was not a separate institution. In Potsdam, of course, it was a separate Prussian state institution. I do not know, when it was built by Gauss, whether then the Gottingen observatory was part of the university or whether it was outside.
I see. Well, that's not important here. Well, we've covered quite a lot. What else about your father? I've run out of immediate questions to ask about him. I haven't asked much about his scientific career.
That I'm not a specialist to really answer.
No particular stories or anything you heard about how he happened to do particular pieces of work.
No, no. In the Gottingen time the main observing program was the beginning of trying to do photometric measurements with a photographic plate. It was very new.
The main observational program that my father undertook in Gottingen was the photometry of stars with a photographic plate instead of with the eye, which led to the discovery of colors of stars in a measurable fashion and the first determinations of a tentative temperature scale. Which, by the way, is I think for the first time put down in those 1905 lecture notes which Born took down.  In that observational work, the instrumentation that my father added to the existing telescope was a special camera built by the janitor of the observatory, who happened to be a carpenter and really the factotum who did everything at the observatory. And since he was a carpenter, that first what we now would call a raster scanning camera (then it was called schrafierkassette, but popularly the Wackelkamera, wiggle camera, because it went back and forth), it was all built out of wood. But at that time it worked. In Potsdam, of course, there were shops, and everything was on a more formal level. By the way, this carpenter who was the janitor and everything at the observatory also was the one to help, presumably, my father with his household. The Hilberts — Mrs. Hilbert particularly — told me of the first dinner that my father gave inviting them and one other couple to dinner. This janitor was to serve the dinner, which just didn't come and didn't come. They had a terribly gay and funny time about it, and finally the potatoes came and the meat, if I remember right, never appeared, and my father was terribly embarrassed. I think by way of describing the fiasco to my grandmother in letters he got strict instructions on how to find a caterer and to repeat the dinner. And Mrs. Hilbert said that they were invited a month later or so to a dinner that was very well served and was terribly boring in comparison to the first one.
You don't know this janitor's name by any chance?
Well, let's see — anything else about your father?
There's one much more serious subject, really two, in connection with my father going into the German army. One is: he wrote a will at the time just before leaving Potsdam, which was the obvious thing to do. That will I think is, from the straight human side, probably one of the writings of my father's that shows the most deep understanding.
That's in the (microfilmed) papers that we have here?
I think so. It was in the papers, and I think we put the original into the papers. There were two particular problems, without putting them necessarily all that explicitly, they're clearly there and are in the will only as advice, not as demands on my mother. The first one was: in case of his death she was to move back to Gottingen. That I think must have had two major reasons in my father's mind — one, to give my mother a surrounding in which she was sure of help through her own family and through the much larger group of Gottingen friends of his. Second, I think according to remarks of my mother, my father did not want us children to grow up in the garrison town that Potsdam was. That is, quite in contrast with what one might think of his voluntarily going into the German army: he did not want us to grow up in a surrounding that was essentially dominated by the military Prussian garrison. So I think both from my mother's point of view, having the insight of the needs of my mother, and what he wanted in surroundings for us children, it was really fantastic advice. And of enormous consequences for us children.
By the way, how many were you?
Three. An older sister. I'm in the middle, and I had a two and a half year younger brother. The brother did not succeed in getting out of Germany under the Nazis and killed himself under the Nazi pressure. My sister got out of Germany before me. In many regards my sister was in temperament and energy and activeness the closest to my father.
Yes. Do you take perhaps more after your mother?
In many regards — in slowness, comparatively in any case, I'm much more like my mother. In the last will of my father there was one other condition — not condition, strong advice to my mother — not to let us children know that my father was Jewish until we were at least 14 or 15. I don't remember exactly how he expressed it, but he explicitly said to do everything to let us grow up as normal to our surroundings as possible.
Was that in fact done?
At what point did you know that your father was Jewish?
I don't know the exact age but it was around the age of 14
So it came as a surprise to you.
Completely. I'd never thought about it.
Was it your mother who told you?
Yes, and made me promise not to tell my younger brother. My sister knew already. At first I was quite surprised and I think a little angry, but then I accepted it.
It's interesting that you could grow up in Gottingen and not know that your father, who must have been a well-known figure, was Jewish. You must have talked to many people.
Yes, we grew up again in a very protected, narrow social stratum and essentially surrounded by friends of the family and friends of my father's. Actually, I grew up really in an enormously spoiled way under those circumstances. Carl Runge, for example, one of the real close friends from that time, permitted me for years, when I was towards the last years of high school, to spend one afternoon a week with him. He got me into mathematics and physics well ahead of what I could get in high school.
He actually undertook to tutor you.
Yes. He was retired then and an older man, and a marvelously fine man. So in that sense, in part as a consequence of the will of my father, we children grew up in a very protected, maybe over-protected, manner.
Did you say there was another point in connection with your father's going into the army or were those the two points?
The other point is, of course, the question of his own decision to go into the army, which was not at all what was expected from a man in his position — indeed was not normally done. But there, according to my mother, it was entirely one thing that determined my father, and that was the conviction that German Jews could not expect to overcome the anti-Semitism if they did not lean over backwards in doing their national duty. It was that reason and that reason only for which he did go into the army.
Your father's Jewishness must have been quite important to him then.
Was he a practicing Jew through the time he married your mother?
No. My grandparents — that is, my father's parents — were in no sense orthodox. They were still entirely Jewish by race, by wish, by their friends; also in belonging to the synagogue but, as far as I know, in nothing that one would describe by the word “orthodox”.
But your father did not become a Christian.
No, but he also did not go to the synagogue.
What sort of religious upbringing did you have, may I ask?
My parents had agreed that since the main surrounding was Christian, that we would grow up in my mother's religion, which was Lutheran. But my mother did not go to church except maybe to a Christmas service or on some special occasion, so we children did not go to church at all. Except in the Lutheran church you go for one year of confirmation, which means religious training — once or twice a week, I forget now, with a very formal entrance into the church at the end. My mother sent us children through that at an early age. Both my older sister and I she sent through at the earliest possible time, because I think she understood that we both were rather lively in our thinking and not easily persuaded to sharp lines of religious thinking. So to get us through the confirmation procedure, I suspect she knew she'd better send us as young as possible. And that turned out right. We both had slight difficulties in really accepting it.
Did you have strong religious feelings, or was it simply a matter of general skepticism?
It was the normal skepticism (isn't that right) of a growing young person.
I think now we're talking more about you, which is proper enough: you grew up entirely in Gottingen except for your summers in Potsdam?
Well, except for my first four years in Potsdam, very soon after my father's death my mother did move back to Gottingen, and I grew up entirely in Gottingen. Well, the plan was that I would go to the university in Gottingen but would spend maybe one or two years away from Gottingen, as was normal for German university students.
Did you do that in fact?
I, in fact, did go to Berlin exactly the year to see the Reichstag burn and Hitler become chancellor (Jan.-Feb. 1933). The fall semester of '32 to '33. After the first semester, things were threatening enough so that my mother asked me to come back. So I was only one semester in Berlin and then for political reasons stayed in Gottingen and actually tried to get my Ph.D. as fast as possible.
I see. It wasn't because you were being threatened as a Jew in Berlin, it was simply as a matter of
Well, the situation became more and more threatening in a general way. My mother to some degree leaned on me during those years. The gravest worry being my younger brother who was not well. He had a heart condition and also a very severe nervous condition. He really suffered under the social dictum that a son of that university stratum must go into a university profession, for which he just was not built, and between that and his physical difficulties, it was very difficult. That was a point that my mother did not overcome, and I did not understand and see through.
Tell me something about your own training. You went to high school in Gottingen, gymnasium?
Yes, the classical gymnasium with nine years of Latin and seven years of Greek.
You could have gone to a Realschule?
I could have. But the traditional style still was strong. Quite a few of the university professors at that time in physics and mathematics maintained that the Realschule students did not turn out to have a head start; that the classical training was just as good a training. I cannot judge that, but the general conservative attitude of my mother's family was that the gymnasium was approved. Actually I was extremely lucky. It was one of the best gymnasiums — all state-run. They were all under the Prussian school system, and it was one of those that were called Musterchule — that is, exemplary schools — which was staffed on the average by substantially better teachers. There was one teacher, Max Carstenn, in particular — a teacher in Greek and German literature — who made an enormous impression on my life.
Is that so? In what way?
He was enormously liberal in his outlook, in the sense that he could describe the attitudes represented in the literature from classical Greek to the sternest Christian writers, and everything in between, in a very persuasive and lively sense. He really got me — after the grammar and the vocabulary once were sort of out of the way — got me enormously enthusiastic. And he was one of the early teachers who understood to not suppress students, even though the discipline he enforced was absolute. You couldn't hear a pin drop in his classes if he wanted it quiet, and most of the time he did. But at the same time he did not suppress us in our participation.
You mean in class you could speak up?
Yes, yes. He gave us themes to write — I mean with a level of freedom, but still a basic theme, that I just enjoyed, even though that was not my strong side, writing them. Also, my sister was only one year ahead of me, we were extremely close, and she got very interested in both Latin and then Greek. That did not exist in the girls' school, neither Latin nor Greek.
Not even Latin.
No, not at that time. It became very expensive and very strenuous to tutor her separately. There an uncle of mine, one of the younger brothers of my father's, who on a visit really sort of took the initiative to make my mother decide to see the director of the gymnasium. In a general way, my uncles — that is, my father's brothers — were not terribly good in understanding my mother and not good in influencing her, but on that occasion this uncle did it just right. My mother was not against it, but she was uncertain — in particular with my father's wish of us growing up as normal as possible — whether that was really the right step to do. But she did it, and my sister passed all the tests.
She came and sat in in the gymnasium?
In the gymnasium one class advanced from me.
This was allowed even though not in the university.
Well, this was much later than the time my mother was put out of the university.
Of course, that's true.
No, by that time the university was entirely open. I mean World War I had happened, and the Kaiserreich had changed into the Weimer Republic. There were many many changes.
Yes, right. But even so your mother had to struggle against the stereotype that a woman didn't.
Oh, yes, and actually in the gymnasium there were very few girls, mostly only girls that came from other towns where they had training in those fields. So this was quite a rare case. It meant that my sister and I had many of the same teachers, and we collaborated very heavily; much of my homework in Latin and Greek was done by my sister — in English, too — and I did her math. The teachers all knew. It was quite open.
Already at that point you recognized that your forte was on the mathematical/sciences side.
There was no question just by the grades — how easily I could get good grades.
You mentioned that you were tutored by Runge in mathematics. Did you have any other of this sorts of relationships before you went into the university? Was there anyone that you discussed physics with or astronomy with?
My mother took me a couple of times to the then director of the observatory, Professor Hans Kienle, who later became my main teacher in astronomy — took me to him already when I was a high school student. These were more or less social visits, but with a little chance of me asking him astronomical questions. By that time I had put together a little telescope and an apparatus to photograph stars with a small box camera, in what free time the school left me. And after a while, Professor Kienle had seen enough of that, that he loaned me from the observatory an old four-inch telescope.
This is still while you were in the gymnasium.
In the gymnasium. It was actually a telescope built by Fraunhofer. I did not, of course, appreciate the historical value except that I was naturally trained to be careful and respectful of things that didn't belong to me. I never enjoyed straight observing, looking at objects, very much, but this question of taking photographs, and photographs through color filters, and really imitating experiments that I had read about that astronomers had made.
Your father's experiments?, or in whatever books you were reading…?
No, I would not read much… oh, introductory or semipopular astronomy books. There were not many then, but I had a couple.
Were you interested in astronomy as long as you can remember? Or was there some point at which you began to go into this?
I myself would not remember. My mother maintained that as soon as I could speak I talked about the stars. That was still in Potsdam. And except for one year when I wanted to be the man who delivered the milk because he had a horse and cart, my mother maintained I always wanted to become an astronomer. I'm sure she consciously tried not to influence me in that direction at all.
Do you think she wanted you to become an astronomer?
She clearly was not unhappy with the idea.
What about the other family — all your father's relatives and your mother's relatives? How did they feel about it?
I really do not know. I know that some of my father's relatives, who were not academic people, felt that my mother drove us children too much towards university careers; and that, I would now agree, was the case. But neither in my sister's nor my case, because we were both good students, was there any active objection that I ever got aware of.
I see. So now at the time that you went to the university, did you already know that you were interested more in the theoretical and the mathematical side than in the instrumental side?
No, I don't think so. I had as a high school student mostly done what was within my means then (isn't that right?), experimenting in a very primitive way with little telescopes. At one time three of us from my school decided to form an astronomy club. I decided or proposed what we should do, which was to develop rather lengthy equations about some spherical trigonometric problem and compute tables. Well, the others didn't have any counter-proposals, so they sort of had to agree. That club lasted one session. That, was the end of my forming an astronomy club. So in some sense I enjoyed the mathematics in astronomy early, but I would not say that I was in any sense decided.
I see. Did your uncle-in-law Emden, did you have much contact with him? Did he have much influence?
That was for me at the time a very disappointing experience. The Emdens lived in Munich and invited me a number of times as a schoolboy for vacations down there, which I enjoyed very much, for Munich has a lot to see. They had a number of children — one son and several girls, a lively family. So purely as a child I liked it there. But I also had the idea I wanted to ask Uncle Emden astronomical questions. And Emden just did not talk in the family. When I asked him a question he would say, "Hunh?" or "Mmm?" And that was as far as it went. You cannot ask many questions and get only yes or no. Whether he really intended to not influence me, and not from inside the family drive me to astronomy, or whether it was a general behavior in the family — I do not know, he hardly talked to his children either…
That was a disappointment not only to you then but to me now. I was hoping you might be able to tell me some inside stories about how the ZEITSCHFRIT FUR ASTROPHYSIK was founded or things like that. Do you know anything about that? Through Emden or others?
The initiative did not come from him.
He was one of the founders, was he not?
I think it was rather more in honor of him than by him. As far as I know the history, one of the key drivers was Grotrion, a spectroscopist and astrophysicist, but essentially after my father's time. He was married to a Gottingen girl from a Gottingen family that my mother's family knew very well. I learned to know personally Grotrion already as a child, and he knew the family very well.
To return to your education then, specifically in Gottingen because I think that's the most interesting thing, I'm curious to know what sort of an education was being given to astronomers in that time and also anything you can tell me about what you may know from before your time. Did you start out learning astronomy? Did you start out learning physics and mathematics? What sort of things…?
That I can describe by one (for me, then) terrible story. The moment I had finished my exam at the high school and was ready to go to the university, I did go by myself to Professor Kienle of the observatory and asked him what courses in astronomy I should take the first semester. He, being a very outspoken and excellently forward man — you knew exactly what he was thinking, in that sense he was an excellent teacher — he told me, "I don't want to see you in the observatory for the next two years. The first thing you have to learn is mathematics and physics. I don't want to see you in any astronomical course or in the seminar here." I was dreadfully disappointed. But I was not trained to argue with authority, and I think by character I was not particularly eager to argue either. He did tell me that I could come to him at the beginning of every semester if I wanted to and he would suggest what physics and math courses I should take. And, as a matter of fact, he advised me extremely well. So essentially for two years I had no contacts with astronomers. In the first year my main math teacher — teacher in the sense of the man, of course — was Courant.
Courant, I see. Well, you can't learn from a much higher source.
As a basic lecturer he was marvelous. His lectures were extremely muddled, as he was not orderly in any way, but somehow the basic ideas were extremely clear even though the detailed steps were quite disorderly. In contrast to another mathematician, Edmund Landau, who gave every alternate year the same introductory course extremely cleanly, and, so they said, one never understood what he was driving at. The contrast couldn't have been larger, and by sheer luck of which year I came to the university I got Courant, which was a marvelous foundation. Otto Neugebauer was then a young assistant, or whatever the title was at the Mathematics Institute, and I learned from him some other things like vector analysis. He was a very stern teacher. His vectors never had as few as three components. But by hard work one could follow. In physics unfortunately I really missed Born entirely and Frank nearly entirely, by way of my preparation just getting ready when by Nazi pressure they left. So there my training was under good teachers but not as outstanding teachers.
What year did you enter Gottingen University?
I think it was '31 but that may be one year off. But I stayed in Gottingen, if I remember right, for three or four semesters, and then went to Berlin for that on half semester and then came back.  After I came back, of course, the university situation rapidly deteriorated and became very complicated. In general relativity, for example, the only course I had was from Professor Heckmann at the observatory, given under the title "Dynamics of Fastmoving Bodies." You could not talk about general relativity anymore.
What was the feeling? Tell me, because I'm particularly interested in that. What was the feeling about general relativity? Or perhaps I should ask: did you encounter general relativity when you were at the gymnasium, were you that familiar with things?
No, I don't think so.
There wasn't any in the sort of introductory texts and so forth, these little introductions to astronomy that you read?
No, they lagged way behind. But astronomy texts, you know — the phenomenon of an Eddington or a Hoyle or a Gamow is very rare. There were none of that kind of books that I remember in my youth.
What about quantum mechanics? When did you first encounter quantum mechanics?
At the university. I mean both those fields were completely accepted (isn't that right?) at the time I was a student.
But you hadn't sort of heard of the ideas before then. I'm interested …
Oh, probably heard, I'm quite sure, in popular lectures that I attended. The university while I was a student often offered lectures. Indeed, I got the permission from the school to attend one or two popular courses at the university, and I'm sure I heard of the basic ideas but it didn't make any impression, probably because I simply didn't understand.
I'm just curious to know what level popularization of relativity and quantum mechanics might have reached at that time. Of course, being in a university town you would have unusual opportunities to hear what was going on.
But I don't think it was then any more considered. Well, it was an exciting subject, but it wasn't, you know, the discovery of the subject.
Yes, it was already quite…
It was a settled and accepted field.
When the Nazis came in there were these difficulties.
Yes. But I think all people in my state accepted that as politics, not as science.
Did you encounter difficulties because your father had been a Jew? I'm talking now while you were an undergraduate. Or was it simply the general disruption of university life in general.
Personally I encountered fantastically little except that I had to get more and more cautious not to make difficulties for my non-Jewish friends, and most of my friends by sheer statistics were non-Jewish.
What kind of difficulties?
Being seen together with a Jew or a half-Jew.
It was known that you were a half-Jew?
Oh, that was quite clear by that time. Indeed, it all had to be documented. Everybody had to get birth certificates, at least of the grandparents, all four grandparents, so as to be classified individually. I must say that the two astronomers, my main astronomical teachers, on whom I depended, of course, in my last years in Germany, which were the most difficult ones, were enormously courageous.
Kienle and Heckmann.
Heckmann, right. Kienle, for example, after I had finished my Ph.D. but not yet found a post-doc outside — I should rather say he had not yet found one for me, because there was nothing really that I could do — I had one observational program unfinished (that was not my thesis) and here.
That was the Cepheid light curve?
The observational program was a small cepheid problem and the other was the theory. The thesis was on the theory of cepheids. But on the observational program I hadn't finished the reductions, the measuring of the registrations. And Kienle Asked the janitor of the observatory to bring the measuring machine at night to my mother's apartment so that I could finish the work, and then again at night had it brought back, which at that time took real conviction. And also, Heckmann teaching the course on relativity, even under another name: there were plenty of people who knew what he was doing.
It was a well-attended course, physicists came and so forth?
Yes, a regular class, though on the other hand — and there I have to include myself — in thinking back it was unbelievable how inactive we were with regard to the Nazis. As far as I personally was concerned, when I think back how I behaved those years in doing exactly nothing in the political arena — except being very interested in following it, not just for personal reasons but also for a broader interest — but I did nothing. I didn't participate in any protests or anything (of which there was very little). But, after all, I was one who was very much involved. When I came to this country I slowly learned what a practicing democracy was about; even though during the Weimer Republic I was very much brought up on the principles of a democracy, as to how one daily behaves in a democracy to make it work, we knew nothing. I had to learn an enormous lot, and not all the easy way, in this country.
I see. We'll have to get back to that later on. But right now I wanted to ask you a little more about the observatory. I'm interested in Heckmann teaching this course to physicists, and I'm interested in what sort of relations there were between people like Kienle and Heckmann with the physicists. From what you say Kienle seems to have been a man with the same kind of feelings as your father for astrophysics. He was a student of Seeliger also, I notice.
Right. He definitely looked at astronomy in the modern sense of astrophysics even though obviously he himself, to some level, trained us in classical astronomy. You still had to compute a comet orbit and learn the truly classical sort of astronomy.
Yes, and the observatory had all these programs of quite classical astronomy, various instrumentation.
Observation, no. The Gottingen Observatory — actually Kienle closed all the meridian circles and went entirely into photometry. There was one astrometric instrument preserved to train us students, and we had to prove that it was in the garden of the observatory and not across the fence, which was only 10 meters away.
Well, that's quite a job.
Yes. No, in that sense Kienle was entirely a modern astronomer, but he was not a theoretician. He, for example, had enough understanding so that at the time that I developed more interest in the theoretical side he suggested that I read Eddington's book on the stellar interior, which was then probably the leading theoretical book, even though it covers only a very definite section of theoretical astronomy.
That was one thing I wanted to ask you: how you got your training. I have here in my hands (your copy of) DER INNERE AUFBAU DER STERNE by Eddington in a 1928 translation.
At the time I was a student, the idea that you learned everything by taking courses was not at all prevailing. As an example I remember going to Professor Kienle at one point to again ask for the courses for the next semester, and he said, "Oh, you have to take the optics course to be given by Born." Actually Born had to leave before he gave it. And I said, "But don't I first have to take a course in electricity and magnetism?" Kienle just looked at me and said, "What do you do on the Easter vacation?" That was a month, and there was no further question. I learned by myself one of the most fundamental fields, on which optics and quantum mechanics and everything else builds. But that was entirely normal. And therefore in the sense of not having courses in theoretical astrophysics but rather being advised, helped, what to read.
So was this essentially your introduction to stellar structure, reading Eddington?
Yes. And, as a matter of fact, there was nobody at the observatory in Gottingen at the time of my thesis who really knew that field and could judge my thesis. It was passed. The moment I sent a copy of the thesis to Eddington, very soon I got a long letter back from Eddington quite definitely proving that I had made one real mistake and that the thesis was wrong, but it was printed by that time.
Do you still have that letter?
Yes, sure I do.
Good. Don't throw it out. These things are interesting. At what point did you start reading the general literature on stellar evolutionary interiors, or in general?
I can't quite remember, but it must have been at the very end as a Gottingen student, probably the last year. But then the moment that I took the next step and got my first post-doc in Norway, then Rosseland made me read. He was himself a theoretician in the field (isn't that right?) and he really put me to read a lot.
I see. So that was when you really began to read. You told me before that you started reading Eddington and you got up to the chapter on pulsating stars, which is definitely more paged than the rest here,  and stopped at that point. How did you come to be interested in stellar interiors, stellar structure, in the first place?
I think it was at that time probably the only field we would describe now as modern theoretical astrophysics, that had real body to it. For example, stellar atmospheres, which was the next field: the very first papers on the curve of growth — how a line intensity in the spectrum depends on physical circumstances and abundances — came out only while I was a student. Indeed, the very first seminar I had to give happened to turn out to be the initial paper on that subject written by Russell in Princeton. Without my having the vaguest idea that I would end up here. That field we really got our introductions to through seminars. But, you know, as the articles appeared.
Were these seminars held at the observatory?
Did the physicists come to them sometimes?
And did you also attend the physics seminars?
Rarely, hardly at all.
And you were there when quantum mechanics was sort of getting finished off and so forth — Dirac and all that. Were you aware that was going on?
Yes, I was aware of the developments, but at the same time I did not study it very hard. Quite minimally in fact. And I should also emphasize that the same is true with general relativity. I essentially took one course and did not study it any further. Also modern statistics, for example, which was just sort of taking its modern shape, I did not study as such. In many ways in that sense I was not as forward-looking as my father would have been at that time, for example. But it still is true (isn't that right?) that even though Dirac put the final intellectual touches to the basic elemental concepts, the philosophical changes were (already) there.
If you could come back one minute more: when you asked me about getting interested in the stellar interior, I think it is perhaps not false to say that Eddington's book was the first book in theoretical modern astrophysics that really had, as I say, body to it; and therefore it was really rather Kienle's idea that "Here is clearly a good book in a major field of theoretical astrophysics, why don't you look into that?"
So you knew in a way that you wanted to do theoretical astrophysics, but it was Eddington's book that showed you a field that you actually could do.
That I had at least easy entrance to by the book. I don't think that Kienle had any other ideas, of choice of a section. In a certain way I always maintain that it was a marvelous example of a very dumb way of choosing one's field. Because after I had read the basic chapters in Eddington's book, it was entirely clear — and Eddington makes it clear — that the real final solution to stellar models and to the life history of a star, stellar evolution, as we now call it, would not be possible until nuclear physics had reached the point where nuclear rates of fusion processes could be known. So to go into a field where the cream had been skimmed by Eddington up to that point where you then were entirely dependent: if and when the physicists could make the next necessary step for that field — to go at that point into that field, in afterthought I still consider a magnificent example of stupidness. And then unbelievable luck, when I was just ready for a professional position and having trained myself in the field, the physicists did present the first approximations to hydrogen fusion; and then (isn't that right?), except for a delay by World War II I was one of those ready to jump into it.
Well, did you perhaps have any means of knowing, even if only unconsciously, what kind of progress the nuclear physicists were making? At that point had you had much contact with nuclear physics?
Not enough to have any estimate of that. I doubt that I even thought that properly through.
Did you recognize at the time from reading Eddington how much it did depend on the knowledge of…?
Yes, and in a way it was natural therefore to go into what you might consider a side issue — not an unimportant one but not the central line of the field, namely the pulsations. There was quite a lot to do there without nuclear processes. But the central line was essentially blocked.
I don't think today we'll have very much time to talk about it, but while we're on the subject did you have any contacts with Eddington. It was, after all, already slightly out of date by that time? At what point did you hear of Stromgren's ideas that there was a lot of hydrogen in the stars? I think he came out with it in 1933.
If I remember right, I may have gotten the first knowledge of that in the year in Oslo. However, most impressively I got in contact with that after I came to this country, and very largely through a very fast and close first acquaintance and then friendship developing with Chandrasekhar, who frequently had me at the Yerkes Observatory where I met Stromgren and where all these things were discussed in full.
We'll get back to that. Let me try to finish off Gottingen with the time we have. I looked up who was at Gottingen and I'm interested: there were quite a lot of people at the observatory actually if you include all the assistants and guests and so on. It must have been a busy place.
It was an extremely lively place, and Kienle was extremely good — even though he himself maybe was not an outstanding astronomer — to recognize good young people. Siedendorf had just gone through and was away; Biermann had been a student there — just out but he visited often — and Biermann was one of the first people who had an active interest and some publication in the stellar interior with whom I ever talked about stellar interior problems.
Did you talk with him regularly or frequently?
No. He was not anymore in Gottingen. I saw him only two or three times at meetings. It was later that we saw each other more often. As a student the habit of going to meetings was quite limited. Financial circumstances were tight and it was not the style.
Within the observatory there was a lot of socializing?
Did Kienle have parties in his apartments?
Oh, yes. The Gottingen Observatory at that time was a very happy place. There were a number of students with whom I got along extremely well. One by the way (von Hoff) developed into a very strong Nazi, and in spite of his becoming a very active storm-trooper and Nazi he and I had one vacation in the Alps together.
You mean after he had become a Nazi.
Yes. It is still hard for me to understand in afterthought how he settled it with his conscience and how I settled it with my conscience. It's quite bewildering. Haffner was just about exactly my age. Later Haffner became the head of the Bergedorf Observatory until some years ago when he decided to go to a somewhat smaller responsibility. So it was a lively and happy place.
I was interested in looking at this. There was a post called the technische assistentin which seems always to have been filled with a fraulein. There was a fraulein Waschmann, a fraulein Mobes, a fraulein Schlieper.  Was this the sort of thing they had at Harvard where they always had a woman doing some particular type of work?
Yes, but at Harvard they had a large number, Harvard was famous (isn't that right?) for a large number. But it was probably the beginning of the style coming to Germany, where the semi-routine things were given to the most educated women.
It was that same type of thing.
Yes, only a much smaller staff and starting much later.
But these women would be there and interacting with the other people and so forth.
Well, usually one at a time.
One at a time, yes. Tell me; there was an Observator, Meyermann. What sort of position was that? What would he be doing?
That was actually I think by official title the highest position after the professor. Geheimrat Meyermann was quite an old man who had been at the observatory already in my father's time and then served in a German observatory on the coast of China — Tsingtao I think was the name. He had had a very hard time in prison during World War I and was called back and given a position in Gottingen. My mother knew the Meyermanns extremely well from the earlier times. As an astronomer he was not outstanding in any way. He was an extremely kind and friendly man and did all the classical teaching. It was he under whom I had to do my orbit computing and so on. He got the position I think as somebody to whom it was felt one owed a position. Otherwise I doubt he would have been the choice.
I see. Another sort of position I'm interested in is the mechanikermeister, Friedrichs.
Well, he was essentially the head of a small shop and played a very important role in the laboratory for absolute photometry which Professor Kienle made the main program of the observatory. He played a central role. He was a good instrument-builder in the classical sense — no electronics or anything of that kind — it was essentially mechanics shop. But, you know, a bit of a designer, that sort of intermediate between a technician and an engineer.
A model-shop man.
Right. And it was he, by the way, who at night brought the instrument to my mother's house.
Who did the electronics? Because you did have some photometers and so forth there by now?
Yes, but that was so new an art that the astronomers did that themselves.
I see. Did you get involved in any of this mechanical design in electronics?
Not in the design but Haffner and I, for example, for a semester or two were put into the photometric laboratory, and Kienle gave us jobs to do what actually needed doing and some that hadn't been done before. He kept us quite alone and let us invent our ways. As a teacher he was very good and he got results.
Was there any feeling that some people there were theorists and some people were instrumentalists, some people were observers?
The majority of the people frowned on that in Gottingen.
Frowned on making that distinction?
That separation was acknowledged as being probably necessary and proper in physics but Kienle's general attitude, and Heckmann very much joined him, was that that division might not turn out too healthy on the average for astronomers. Even though they accepted, for example, that Biermann refused to do any observational or instrumental work, and many stories were told by us students of how clumsy Biermann was with running an instrument. He was the one pure theoretician.
I see. What about Heckmann himself? Of course he had some mixture of the two, didn't he?
Very much so. He was a first-class observationalist, maybe not as an instrument-builder. He did some very fundamental pioneering work in the photometry of clusters, which later on (I did not know it then) became so fundamental for stellar evolution.
You weren't familiar with that work at that time?
Oh, I was very familiar with it, but I didn't realize how much I was going to use it later on. It was a great advantage. Indeed, we students in turn assisted him and others in observing.
What was your relationship with Heckmann?
A very happy one. I liked him very much and I think he liked me, even though the difference was large.
He was ten or twelve years older than you.
Yes, but the status was what counted. But I remember very well one evening we walked up to the Heinberg where the new refractor — I forget how big it was but maybe a 10-inch or something — was just built, for me to assist him observing with the refractor. It was quite icy out and he was a mountain climber and had crampons on; I didn't. I had a terrible time keeping up with him, but at the same time this gave me a three-quarters' hour chance of asking him infinite questions that I was a little uneasy taking up his or Kienle's time with. So all during the way, mostly out of breath, I got really good answers to a lot of questions on astronomy. But I also remember that at the next seminar that he gave, in front of everybody else, after he was finished, it was time for questions and then Heckmann looked at me and said, "Why don't you ask your questions now and not only in the dark when we-are climbing up to the Heinberg?" Of course, I was ready to creep into the nearest mouse hole. But on the whole we had an extremely friendly relation, and I learned an awful lot from him. He was different from Kienle. Kienle could be quite rude. These seminars were really rough, and the language sometimes used by Kienle you didn't use in front of ladies. And when Kienle made up his mind that a student was not Ph.D. material — you see, there were essentially no exams at that time — the way Kienle did it was very brutal but very effective. He gave the student a topic to give a seminar about that he was fairly sure he couldn't handle, and then he would slaughter him right there in the open in the seminar. It was rather cruel, and I hated a couple of sessions where I saw my young colleagues essentially being kicked out. But at the same time it was extremely difficult because there were no exams to terminate a student. And for the student it was much better (isn't that right?) to be terminated when he really clearly had no chance. So life was not all that polite and that easy. Indeed, the students considered the first seminar assignments, or colloquium assignments, as quite the equivalent of being very high-level exams, with the whole observatory there. But at the same time they were exciting. I mean, normally they were not slaughter sessions.
Was the same thing going on in the physics seminars, do you know?
That depended enormously on the different people. I'm quite sure that neither Born nor Frank would do it in that style. Whether Pohl, for example, who was also quite rough, an experimental physicist, might do it more like Kienle, I really do not know.
I have a few other names here. I don't know whether they mean anything to you. If you had any particular relationship with them, I'd like to know about it. There was Wildt, of course. I'm only talking now about relations you may have had in your Gottingen days: Wildt, Strassl, Wempe.
To start with, Wildt I knew personally quite well. He was assigned by Kienle to teach me my first astrophysical observing. He was a step above me, but to the degree that it was permitted we really became quite close friends, and he played a very fine and helpful role in my coming to this country. He came to this country a few years ahead of me and actually spent his first years here in Princeton as a sort of post-doc.
He went to Mount Wilson also at some point in there.
Yes. Excuse me, maybe the first year was in Mount Wilson, and then Russell gave him some years here.  I remember the first letters that he wrote to the observatory from Princeton. We all thought it terribly funny: "We eat in a gothic chapple while the grammophone plays Beethoven" — three incompatibles in one.
Why did he leave Germany? This was in 1935.
He was — I don't know whether formally, but technically — engaged to a Jewish girl. Also, he was extremely sensitive anyhow. I don't think, even without the question of the girl, he would have stood the Nazis. He did not get to marry that girl, and that was quite a tragic story by itself. I think he was in many ways the most gifted of the three names that you mentioned, and while he was here in Princeton of course he did extremely fundamental work on H minus. Strass and Wempe both even then I did not consider as terribly exciting, though very friendly, persons. But essentially assistants to Professor Kienle in the observatory program of photometry. I knew them both well and spent many hours, both personal and scientific, together. Indeed, both of them had offices where they had to come through the library where we students had our desks. All day studying and talking was mixed up.
One more thing I wanted to ask you, to take a few more minutes about Gottingen: I'm curious particularly about cosmology and about the ideas that people had about cosmology and what sort of feelings people had in Gottingen. Just to start off, I noticed in the 1936 report of the observatory it said that Heckmann had done theoretical work on the possibility of explaining expansion somehow through classical mechanics. Now, that's just a starting point, and I'm just curious about what Heckmann and Kienle may have thought about expansion, about general cosmological questions.
I think I can answer those questions only very vaguely. The interest was very active, and we students loved to hear, particularly from Heckmann, who was very ready and very talkative — occasionally he would just come into the library to show us something because he had to show it to somebody. Particularly Heckmann, thoroughly trained in general relatively, was wondering which of the phenomena that had observationally then appeared in astronomy really needed general relativity, and to what degree one could think also in classical terms. I think that was strongly the motivation in that regard.
I see. Of course, this is 1936, so it's a good thing to put in the report anyway.
Yes, though I doubt that that was… I would be surprised if that was politically motivated.
I see. In any case, he was, as you have already said, a strong believer in general relativity.
Oh, yes. But the real understanding of relativity was really needed and differed from classical ideas. The confusion was still enormous. All these so-called paradoxes: the books were full of them. To disentangle them just took decades. And in part I feel very strongly that it was Einstein's own insistence in maintaining that every coordinate frame was equal to every other that made much of the confusion. It was a concept that was absolutely vital for his conceptual development. But then when you put it to other people of less insight, it became quite confusing. Because it is true that one can write the relativistic equations in any coordinate system, but the values of the terms, the coefficients in the equations are completely different, are miserably complex in some frames of reference but are as simple nearly as Newtonian in some systems. There was I think one short spell when Einstein at least admitted the term "convenient frames of reference." He never permitted the use of "inertial frame of reference," even though by now (isn't that right?) everybody agrees that there is something equivalent to the classical, inertial frame which is in fact the one that is normally used in astronomy. In that sense these questions that Heckmann was interested in were, I think, very essential to clear one's mind: for what do you need general relativity, for what, particularly of the new astronomical observations, the classical frame or the relativistic conceptual frame was needed.
Was it generally accepted in Gottingen by that time that the Hubble expansion did occur, that there was a Hubble relationship and so forth?
Yes. At least I never knew the universe other than expanding.
And also the other question about the other galaxies, as we now call them, being external galaxies and not small things inside — that was all settled.
Settled, yes. So what would have been the cosmological questions that were being debated at that time? Were there any strong debates that stuck out?
Oh, the whole question (isn't that right?) of the closed or open universe was as active then as now, on much less basis. Not that our basis now …
I see, but people still were very interested?
Oh, yes. I mean whether the volume was in fact finite attracts my liberal arts students in the beginning astronomy course just as actively as it did then.
I see. Okay, I think that's a good place to stop for now.
Karl Schwarzschild Papers. Microfilm at AIP Center for History of Physics, NY; Princeton Univ. Library; American Philosophical Society Library, Phil.; Bancroft Library, Univ. California, Berkeley.
Sommerfeld was in Gottingen 1893-97, in Munich from 1906; K. Schwarzschild was in Munich 1893-96, 1899-1901.
In the K. Schwarzschild Papers.
This would make the entrance to Gottingen in 1930.
The edges of the pages of this chapter are darker, from being used more.
See Gottingen reports in VIERTELJAHRSCHRIFT DER ASTR. GESELL.
Wildt was at Mt. Wilson 1935-36, then at Princeton 1936-42 (visiting lecturer at Harvard, 1938).
 In the VIERTELJAHRSCHRIFT.